The front derailleur only moves about 1 centimeter side to side. But that motion, when tuned correctly, makes a world of difference in a bike’s shifting performance. The instructions below will teach you to install and properly align a front derailleur on a road bike. The information applies to most drivetrains (some exceptions are in 11- and 12-speed derailleur positioning). Making these adjustments are best done after giving the equipment a good cleaning.
Adjust the derailleur orientation
- Position the height of the front derailleur so that, at its closest point, the outer cage passes 1–2mm (1/16 to 1/8 inch) above the outer chainring.
- Position the outer plate of the derailleur cage parallel to the chainrings or to the chain in the lowest and highest gears when viewed from above. When on the inner (smallest) chainring and largest cog, the inner cage plate should be parallel with the chainring or the chain.
- Similarly, check this by shifting to the big chainring and smallest cog and sighting from the top.
A note about derailleur position: The lower edge of the derailleur outer cage plate should roughly follow the curve of the chainring, though the tail of the plate will generally be a bit farther above the chainring than will the leading edge. However, if the tail of the cage is way above the chainring when its leading edge is the correct 1–2mm above it, the derailleur is not properly matched to the bike and chainring and may not shift well. This can happen due to trying to mate a front derailleur (especially an older one) designed for large chainrings (39–53-tooth chainring pairs or the like) with a “compact” crankset (one that accepts smaller chainrings, like 34–50T).
The front derailleur has two limit screws that stop the derailleur from throwing the chain to the inside or outside of the chainrings.
These are sometimes labeled L for low gear (small chainring) and H for high gear (large chainring).
On most derailleurs, the low-gear screw is closer to the frame. If in doubt, you can determine which limit screw controls which function by trial and error. Shift the chain to the inner ring, and then tighten one of the limit screws. If turning that screw moves the front derailleur outward, then it is the low-gear limit screw. If turning that screw does not move the front derailleur, then the other screw is the low-gear limit screw.
Low-gear limit-screw adjustment
- Shift back and forth between chainrings.
- If the chain drops off the inner ring to the inside, tighten the low-gear limit screw (clockwise) one-quarter turn and try shifting again.
- If the chain does not shift easily onto the inner chainring, loosen the low-gear limit screw one-quarter turn and repeat the shift.
High-gear limit-screw adjustment
- Shift the chain back and forth between chainrings.
- If the chain jumps over the big chainring, tighten the high-gear limit screw one-quarter turn and repeat the shift.
- If the chain is sluggish going up to the big chainring or does not go up at all, loosen the high-gear limit screw one-quarter turn and try the shift again.
- Pedal hard in your highest gear (big chainring/ smallest cog). If the chain rubs the front-derailleur outer cage plate, unscrew the high-gear limit screw slightly.
- With the chain on the inner chainring, remove any excess cable slack. Turn the barrel adjuster on the cable stop or along the cable housing counterclockwise (or loosen the cable-fixing bolt, pull the cable tight with pliers, and tighten the bolt).
- Check that the cable is loose enough to allow the chain to shift smoothly and repeatedly to the inner chainring.
- Check that the cable is tight enough that the derailleur starts to move as soon as you move the shifter. Fine-tune while riding.
- Pedal hard in your lowest gear (small chainring/ biggest cog). If the chain rubs the front-derailleur inner cage plate, loosen the cable tension slightly. You may also need to unscrew the low-gear limit screw slightly if it, rather than the cable, stops the front derailleur’s inward movement before it clears the chain.
Adapted from Zinn & the Art of Road Bike Maintenance by Lennard Zinn, with permission of VeloPress.